From May 20th, 2022 to October 26th, 2022, 100% user give 5-star rating. for Gmail HTML Editor chrome extension.
Does exactly what it says on the tin - allows me to edit the raw HTML of a G Mail, including adding HTML from other sources. Works
Works great - just needed to reload gmail page to get it to show up. FYI, doesn't show up in tabs where gmail was already open when the extension was installed. Worst case, try restarting your browser.